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.Dd May 19, 2024
.Dt LINUX 4
.Os
.Sh NAME
.Nm linux
.Nd Linux ABI support
.Sh SYNOPSIS
To enable the Linux ABI at boot time, place the following line in
.Xr rc.conf 5 :
.Bd -literal -offset indent
linux_enable="YES"
.Ed
.Sh DESCRIPTION
The
.Nm
kernel module provides limited Linux ABI (application binary interface)
compatibility, making it possible to run many unmodified Linux applications
without the need for virtualization or emulation.
Some of the facilities provided are:
.Bl -bullet
.It
Linux to native system call translation
.It
Linux-specific system calls
.It
Special signal handling for Linux processes
.It
Path translation mechanism
.It
Linux-specific virtual file systems
.El
.Pp
The path translation mechanism makes Linux processes look up file paths
under
.Va emul_path
(defaulting to
.Pa /compat/linux )
before
.Pa / .
For example, when a Linux process attempts to open
.Pa /etc/passwd ,
it will first access
.Pa /compat/linux/etc/passwd ,
falling back to
.Pa /etc/passwd
if the compat path does not exist.
This is used to make sure Linux processes load Linux shared libraries
instead of their similarly-named FreeBSD counterparts, and also
to provide alternative versions of certain other files and virtual
file systems.
.Pp
To install Linux shared libraries and system files into
.Pa /compat/linux ,
either use the
.Pa emulators/linux_base-c7
port or package,
or
.Xr debootstrap 8
installed from
.Pa sysutils/debootstrap .
.Pp
To avoid mounting Linux-specific filesystems at startup, add the following
line to the
.Xr rc.conf 5
file:
.Pp
.Dl linux_mounts_enable="NO"
.Sh SYSCTL VARIABLES
The following variables are available as both
.Xr sysctl 8
variables and
.Xr loader 8
tunables:
.Bl -tag -width indent
.It Va compat.linux.debug
Enable debugging messages.
Set to 0 to silence them.
Defaults to 3.
A setting of 1 prints debug messages, tells about unimplemented stuff (only
once).
Set to 2 is like 1, but also prints messages about implemented but not tested
stuff (only once).
Setting it to 3 or higher is like 2, but no rate limiting of messages.
.It Va compat.linux.default_openfiles
Default soft openfiles resource limit for Linux applications.
Set to -1 to disable the limit.
Defaults to 1024.
.It Va compat.linux.emul_path
Path to the Linux run-time environment.
Defaults to
.Pa /compat/linux .
.It Va compat.linux.osname
Linux kernel operating system name.
Defaults to "Linux".
.It Va compat.linux.osrelease
Linux kernel operating system release.
Changing this to something else is discouraged on non-development systems,
because it may change the way Linux programs work.
Some versions of GNU libc are known to use different syscalls depending
on the value of this sysctl.
.It Va compat.linux.oss_version
Linux Open Sound System version.
Defaults to 198144.
.It Va compat.linux.preserve_vstatus
When set to 1, it prevents Linux applications from resetting the
.Xr termios 4
VSTATUS setting.
From a user perspective, this makes
.Va SIGINFO
work for Linux executables.
Defaults to 1.
.It Va compat.linux.setid_allowed
Enable handling of set-user-ID and set-group-ID mode bits for the new
process image file when image is to be executed under Linux ABI.
When set to 0, new Linux images always use credentials of the program
that issued the
.Xr execve 2
call, regardless of the image file mode.
This might be reasonable or even required, because
.Fx
does not emulate the Linux environment completely, and missed features
may result in security vulnerabilities.
Defaults to 1.
.It Va compat.linux32.emulate_i386
In the x86_64 (amd64) world enable the real i386 Linuxulator behavior.
For example, when set to 0, Linux uname -m will return "x86_64" even if
uname itself is a i386 Linux executable. When set to 1, Linux i386
uname -m will return "i686".
Defaults to 0.
.El
.Sh FILES
.Bl -tag -width /compat/linux/dev/shm -compact
.It Pa /compat/linux
Linux run-time environment
.It Pa /compat/linux/dev
device file system, see
.Xr devfs 4
.It Pa /compat/linux/dev/fd
file descriptor file system mounted with the
.Cm linrdlnk
option, see
.Xr fdescfs 4
.It Pa /compat/linux/dev/mqueue
symbolic link to a mqueuefs mount, see
.Xr mqueuefs 4
.It Pa /compat/linux/dev/shm
in-memory file system, see
.Xr tmpfs 4
.It Pa /compat/linux/proc
Linux process file system, see
.Xr linprocfs 4
.It Pa /compat/linux/sys
Linux kernel objects file system, see
.Xr linsysfs 4
.El
.Sh SEE ALSO
.Xr brandelf 1 ,
.Xr fdescfs 4 ,
.Xr linprocfs 4 ,
.Xr linsysfs 4 ,
.Xr mqueuefs 4 ,
.Xr pty 4 ,
.Xr tmpfs 4 ,
.Xr elf 5
.Sh HISTORY
Linux ABI support first appeared for i386 in
.Fx 2.1 .
Support for amd64 binaries first appeared in
.Fx 10.3 .
Support for arm64 binaries first appeared in
.Fx 12.0 .
.Sh BUGS
Support for some of the Linux-specific system calls and system call arguments
is missing.
